Apparatus and processes for forming wires, sheets, strips, flakes, etc. by contacting molten metal with a rotating disk or drum are well established in the field and U.S. Pat. Nos. 4,705,095 and 4,813,472 are illustrative of techniques used to achieve the desired result. The terms ribbon, sheet and strip are often used interchangeably.
U.S. Pat. No. 4,705,095 deals with the structure of a casting surface on a rotating drum which is configured to allow relatively fast rotation of the drum for the rapid solidification of a cast metal ribbon sheet. That invention is directed to a knurled surface for the casting area on the rotating drum which prevents air bubbles from being trapped between the molten metal and the casting surface as the drum rotates. U.S. Pat. No. 4,813,472 is directed to melt overflow structures for feeding the molten metal to the cylindrical casting wheel.
A problem exists in the formation of metal strip in the fashion disclosed by these patents and others in the field. The problem is the formation of rippled or wavy edges along the opposite sides of the strip resulting from the rapid freezing process. Often ripples are so acute that the finished product must be passed between pinch rollers to smooth the surface. This is obviously time consuming and expensive. In an attempt to solve this problem, Applicant has theorized the ripples result from differential cooling of the metal transversely across the strip.
Casting drums or wheels used in the rapid solidification industry are traditionally formed to be hollow and within the hollow cavity is water or some other cooling fluid. The cooling fluid serves as a heat sink to draw off heat from the casting surface to keep it at a temperature sufficiently low to freeze the molten metal almost immediately upon contact.
It is accepted heat transfer theory that application of heat at a point on a body results in heat flowing in all directions in the body until the body achieves near uniform temperature. With the rotating casting surface, the edges of the cast metal sheet are believed to cool much faster than the central portion of the strip. Heat from the edge areas of the strip flows through the drum, both radially inwardly toward the water and transversely parallel to the metal cylindrical surface of the drum toward the ends thereof. Conversely, heat extracted from the center region of the strip flows essentially only radially inwardly toward the water in the hollow drum. Along most of the width of the strip being cast, heat conduction is confined principally to radial heat flow through the casting drum with edge effects near the edges where there is a progressively increasing transverse component of heat flow. The result is that the edges freeze first. Then the central portion freezes undergoing the usual contraction during cooling which longitudinally compresses the ribbon, creating residual stresses and compressing the already cooler edges into a wavy pattern.